The greatest risk to human health is neither communicable nor non-communicable disease, it is climate change. Saying this, as I and others have started doing at conferences, seems to take a certain courage. We’ve been emboldened by clear statements from WHO’s director general Margaret Chan and from the Lancet (www.thelancet.com/climate-change). But this week, at a meeting hosted by the BMJ in collaboration with an extraordinary alliance of organisations (http://climatechange.bmj.com, doi:10.1136/bmj.d6775), it became clear that we are going to have to get braver still.

Let me begin by acknowledging that putting climate change at the top of the list of things to worry about is hard when faced with the daily challenges of clinical care: supporting the family of a suicidal person (doi:10.1136/bmj.d5801), advising a woman with polycystic ovaries about the possible outcomes of a pregnancy (doi:10.1136/bmj.d6309), telling a young person he or she has maturity onset diabetes (doi:10.1136/bmj.d6044), or treating asylum seekers while under pressure not to do so (doi:10.1136/bmj.d6637).

But to the top of your list climate change must go. The meeting of over 300 delegates from healthcare, the military, climate science, industry, business, and politics, heard frightening news that none of us want to hear, made more frightening by the measured way it was delivered. As Lord Michael Jay said in his opening remarks, there is no need for hype; the cold hard science is scary enough. Chris Rapley, former director of the Science Museum and head of the British Antarctic Survey, told us that “the science is overwhelming and settled.” His conclusions were unflinching: “Is the planet warming? Yes. Is it us? Yes. Does it matter? Yes. Must we do something about it? Yes.”

To prevent catastrophic climate change, global temperatures will need to rise by less than 2°C above pre-industrial levels by 2100. At or above such temperatures, health and military experts painted a picture of economic and social breakdown, with death and disease on a massive scale caused by resource shortages, migration, and conflict (BMJ 2011;342:d1819). Business as usual will raise temperatures by 5°C. Even if all current pledges on carbon emissions are met, we will hit 4.3°C. The consequences of such global temperatures are unsurvivable. A statement released at the meeting calls for governments to aim for a safer, lower rise of 1.5°C by 2100 (doi:10.1136/bmj.d6760). But how on earth to achieve this?

Professor Hugh Montgomery of University College London, who instigated the meeting, concluded with stark honesty: “What can we do? I don’t know.” The UK’s Climate and Energy Security Envoy, Rear Admiral Neil Morisetti, agreed that there was no clear answer but that we now need to own the problem and the solutions, both as individuals and at an institutional level. “No more talk of them and us,” he said. The meeting concluded that we must give politicians the ammunition they need if we are to create radically different ways of living, and we must develop better narratives to articulate the health and economic benefits of tackling climate change. An editorial this week takes up the challenge (doi:10.1136/bmj.d6520).